employees.js

function Node(data) {
  this.data = data;
  this.children = [];
}

class Tree {
  constructor(rootData) {
    this.root = new Node(rootData);
  }

  add(data, toNodeData) {
    const parent = this.find(toNodeData);
    parent.children.push(new Node(data));

    return this.root;
  }

  find(nodeData) {
    const queue = [this.root];

    while(queue.length) {
      const node = queue.shift();
      if (node.data === nodeData) {
        return node;
      }

      for(let i = 0; i < node.children.length; i++) {
        queue.push(node.children[i]);
      }
    }

    return null;
  }

  getReportStack(name) {
    let finalStack;
    const recurse = (node, stackChild) => {
      stackChild.push(node.data);

      if (node.data === name) {
        finalStack = stackChild;        
      }

      for (let i = 0; i < node.children.length; i++) {
        recurse(node.children[i], [...stackChild]);
      }
    }

    recurse(this.root, []);

    return finalStack;
  }
}

const input = [
  'Sarah',
  'Fred',
  'June Tom',
  'Tom Nathan',
  'Tom Sarah',
  'Nathan Qing',
  'Nathan Fred'
]

const main = (input) => {
  const firstSelected = input.shift();
  const secondSelected = input.shift();
  const root = input[0].split(' ')[0];
  
  const tree = new Tree(root);
  input.forEach(vertexStr => {
    const vertex = vertexStr.split(' ');
    tree.add(vertex[1], vertex[0]);
  });

  const first = tree.getReportStack(firstSelected);
  const second = tree.getReportStack(secondSelected);

  if (first.length < second.length) {
    console.log(first[first.length - 2]);
  } else {
    console.log(second[second.length - 2]);    
  }
}

main(input);